Updates automatically as you type.
Calculated in your browser; your inputs are not sent to us. Privacy
Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
This tool runs in your browser. Your calculator inputs and results are never transmitted to us or to ad/affiliate partners. Ads and sponsored links may set third-party cookies to serve and measure them, but they never receive your calculation values. If you explicitly save a scenario, its permitted fields stay in local browser storage until you clear them. See our Privacy Policy.
Use this result
Share the current inputs or ask ChatGPT to explain the calculation in context.
More options
What does this calculator estimate?
Percent difference compares two independent measurements on a symmetric scale: |A − B| ÷ average × 100. Two readings of 180 and 220 differ by 20.00% — the same 20% whichever value you list first.
- Percent difference is directionless by design — it cannot tell you which value is larger, only how far apart they are.
- Percent change (before → after) and percent error (measured vs known-true value) are different questions with different formulas.
- The tool is undefined when both inputs are zero — there is no meaningful average to divide by.
Percent difference vs percent change vs percent error
The three measures are constantly confused because they all produce a percentage from two numbers. Percent difference treats both values as equal partners — two independent measurements of the same quantity — and divides by their average. Percent change has a direction: it divides the difference by the ORIGINAL value, so 180 → 220 is +22.2% while 220 → 180 is −18.2%. Percent error divides by the accepted or true value when one measurement is known to be the reference.
Pick by question: 'how far apart are these two instruments?' → percent difference. 'how much did it grow?' → percent change. 'how far off is my measurement from the standard?' → percent error.
Why the average denominator
Dividing by A alone would make the answer depend on which value you call A: 40/180 = 22.2% one way, 40/220 = 18.2% the other. Averaging gives one agreed answer for a pair of values with no natural 'first' value — that is what makes the measure fair for comparing two instruments, two labs, or two estimates.
The trade-off is asymmetry blindness: a 20% difference between a hand-measured 180 and a caliper-measured 220 tells you nothing about which one is off. When one value is a trusted reference, switch to percent error instead.
Boundary behavior
- Both inputs zero: undefined — the average is zero, and the tool reports an explicit 'Both zero' message rather than dividing by it.
- One input zero: defined. e.g. A = 0, B = 100 gives |0−100| ÷ 50 × 100 = 200%.
- Opposite signs: the average can be small or zero even with large values (−100 and 100 average to 0), producing an extreme or undefined result — a sign the symmetric measure is the wrong tool for those numbers.
- Negative values: the absolute-value difference is fine, but interpret with care; percent difference on signed quantities is rarely meaningful.
How this calculator works
Formula
Percent difference = |A − B| ÷ ((A + B) ÷ 2) × 100. The denominator is the average of the two values, which makes the measure symmetric — swapping A and B changes nothing.
Worked example
Two labs measure the same part as 180 mm and 220 mm. Difference is 40, average is 200, so percent difference = 40 ÷ 200 × 100 = 20.00%.
Assumptions to verify
- Both values are measurements of the same quantity with no known 'true' reference among them.
- The symmetric (average-denominator) convention is used, matching most statistics references.
- Both-zero input is reported as undefined rather than approximated.
Frequently asked questions
What is the percent difference formula?
Absolute difference divided by the average of the two values, times 100: |A − B| ÷ ((A + B)/2) × 100. Both values get equal weight.
How is it different from percent change?
Percent change divides by the starting value and carries a sign (growth or decline). Percent difference divides by the average and has no direction — 180 vs 220 is 20.0% either way, while the change from 180 to 220 is +22.2%.
When should I use percent error instead?
When one of the two numbers is a known or accepted value. Percent error = |measured − true| ÷ true × 100 — the denominator is the reference, not the average.
Why is my result over 100%?
That is normal for values far apart. A = 0, B = 100 gives 200%. Percent difference is unbounded — it just measures relative separation.
Does the order of inputs matter?
No. |A − B| and the average are both symmetric, so swapping the two values gives the identical result.
Why do both zeros give an error?
The denominator is the average, which is zero — division is undefined. The calculator shows an explicit message instead of a fake number.
Can I use it with negative numbers?
The math runs, but interpretation gets shaky — opposite-sign pairs can average near zero and explode the result. For signed quantities, percent change is usually the right measure.
Cite this tool
BoringToolsKit. “Percent Difference Calculator.” boringtoolskit.com/percent-difference-calculator/ (reviewed 2026-09-20). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Any future sponsored recommendation or advertisement will be clearly labeled and kept separate from the calculation.